Question

Write short notes of chapter sound and explain with diagram where needed.Also do intext questions. class 8th Science please help me​

Answer 1

Answer:

Hey I am grade 7 I take digital notes so hope this helps.  plz mark me as brainliest. I just copy pasted this from my original notes so don't worry did not take me long time to type it bye

Explanation:

The infrasonic sound is the sound of frequency less than 20 Hz & is not audible by human beings but elephants can hear it.

 

Dogs and elephants can hear infrasonic sound.

 

Sound with frequency more than 20,000 Hz is also not audible by human beings & is ultrasonic sound.

 

Ultrasonic sound can be heard by bats & dolphins.

 

Objects travelling in air or any other medium with a speed more than the speed of sound in that medium is called supersonic sound.

 

Jet planes travel fast than the speed of the sound.

Mac Number is a number which tells us how many times the speed of sound is the speed of that object.

Ex. Train monorail  travels 1.5mac what is it’s speed in m/per second?

     330 into 1.5

If you are in a big empty hall and if someone calls your name what do you get?

The sound echo.

Echo is the phenomenon of hearing the reflected sound by a distant obstacle after the original sound subsides. So as to clearly distinguish between the original sound and the reflected sound the reflected sound must reach the receiver minimum for 0.1 sec.

The sound remains in your ear 0.1 second .

The phenomenon of echo can used to determine the speed of sound.

 

Time after which reflected sound is heard=T

Speed of sound = Distance travelled

                               Time Taken

If speed of sound in air is 340 m/per second

Minimum time= 0.1 sec

Minimum distance?

v= 2d

    T

 

= 340 into 0.1

       2

= 17m

To hear a clear and distant echo the distance from the source of sound from the reflector or reflecting surface must be at least 17m .  

Why SONAR uses ultrasonic sound?

Ultrasonic sound is more than 20000 Hz high frequency ultrasonic sound have high penetrating power and they do not get absorbed by a medium.

Frequency= No. of complete oscillations in 1sec

Let the time period be T

Implies In T sec the body completes 1 oscillation

Implies In 1 sec the body completes 1

                                                                T

Implies it is no. of complete oscillations in 1 sec

Implies 1= F(Frequency)

             T                                            

Characteristics of sound

Loudness

Pitch

Quality or Timbre

distance = speed × time.

time = distance ÷ speed.

Speed= distance divided by time

Bigger the surface area larger the sound

Loudness is inversely proportional to the square of the distance. /amplitude

Therefore,  Loudness depends on

                  Amplitude of vibration

                  Source of sound

                  Distance from the source

                  Density of the medium

                   No. of other object located in the

                   room

Why is audible sound not used in SONAR ?

Ultrasonic waves r used in SONAR becoz they can penitrate water through a long distance while audible sound does get absorbed and it cannot travel a long dist.

Conclusion. We use ultrasonic sound in sonar because it has high penetrating power.

The flatness and shrillness of the sound are described as pitch

Loudness quality pitch are not physical quantities becoz they cannot be measured so they are the characteristics of sound.